[Footnote 12: _Science_, March 29, 1901, p. 490.]

[Footnote 13: J.G. Adami, "Principles of Pathology," pp. 641-642.]

As implied by these quotations, a constant and progressive
differentiation of cells prevails in the developing embryo; and when
complete, certain groups of cells act as specialists in doing only
certain kinds of work for the body. These cells maintain their specific
characters in a very remarkable degree under normal conditions. Under
various abnormal conditions, however, these cells may become modified as
to functions, so that cells or tissues of one type may assume more or
less completely the characters of another type. "But," as a very high
authority declares, "the limitations in this change in type are strictly
drawn, so that one type can assume only the characters of another which
is closely related to it. This change of one form of closely related
tissue into another is called _metaplasia_....

"When differentiation has advanced so that such distinct types of tissue
have been formed as connective tissue, epithelium, muscle, nerve, _these
do not again merge through metaplasia. There is no evidence that
mesoblastic tissues can be converted into those of the epiblastic or
hypoblastic type, or vice versa_."[14]

[Footnote 14: Delafield and Prudden, "Text-Book of Pathology," pp. 62,
63.]

This modification of function among the cells which sometimes goes on in
the developing embryo, or under pathologic conditions, is very closely
analogous to the variation which goes on among species of animals and